Modulation of the laser beam is desirable in many laser systems. In ultrafast lasers, it is often of interest to have an ability to select individual pulses, burst of pulses, to reduce the pulse repetition rate and to switch on and off the laser beam with extremely short rise and fall times. It can also be desirable to modify the amplitude of a pulse to a selected non-zero value, or to otherwise affect a signal. In CW lasers, it can be of interest to gate the output, for example to turn on and off the laser beam or to add burst mode operation
The MOPA is an attractive configuration of laser technology for producing high average output powers. The MOPA is often configured as an all-optical-fiber laser, where a fiber-based oscillator seeds a fiber-based optical amplifier. However, MOPA's are also designed as a fiber-oscillator seeding a solid-state optical amplifier, a solid-state oscillator seeding a solid state amplifier or a solid-state oscillator seeding an optical fiber amplifier. MOPA's can also exist as semiconductor oscillators and amplifiers and combinations with fiber- and solid-state amplifiers.
In a MOPA configuration, it is possible to modulate the pump source of the optical amplifier in order to have some method of switching on and off the beam and/or modulating the power of the amplifier output. Direct modulation of a fiber or solid state amplifier is limited in the switching speed due to carrier lifetimes within the amplifier gain media.
For low pulse repetition rates, the duty cycle of the pulses can be insufficient to saturate the amplifier, resulting in the amplifier producing high noise levels in between pulses. In this situation, the amplifier gain can become too high and unsustainable leading to catastrophic damage typically by self-Q-switching.